That did not sound like PMD. FWIW, my experience with PMD was way back in 2002. I wrote an article about it here:
Diagnosis and Treatment of Pinched Mantle Disease in Giant Clams.
Introduction It was almost 20 years ago when I first introduced the Fresh Water Dip (FWD) protocol for the treatment of Pinched Mantle Disease (PMD). Over the years, many reefers altered the protocol somewhat but, on the whole, it still is the only tried and true way to treat PMD, a universally...www.reef2reef.com
Were your clams healthy and with growth until they died? If they were growing and then died, then they were killed by something. One per day sound like predation, especially if they are doing great until they died. If they were expanded but not growing then something killing them slowly. I am sure you though about parasitic snails. Water chemistry is unlikely to be the cause unless you really neglect your tank.
PMD does not araise de novo in your tank. IME, I do not have a long asymptomatic phase, and does not have a dormant phase in our tank.
Clams were all healthy and had great growth! I had such great luck with them even the small ones that were about 1 1/2" grew well for me. I fed DT's Phyto and they were kept under a 400W Metal Halide with dual pc's.
Never noticed anything wrong with any of them until the day the first one died. All were open and fully extended every day. One morning one was sunken in. I removed it and the next day, same thing. It definitely was not pinched mantle and I could not find anything in any article or books I have about clams to explain it. Odd that my blue squamosa never seem affected by whatever it was. When I was dealing with the fireworm, that left a huge amount of a slime covering the clam. I never saw anything on them that I could attribute their death to. I always refer to it as the "clam plague" since I could never find a definitive cause. When I tried to move the other ones to my other tank, I started the same plague in that tank with those clams as well which is why I didn't think there was a predator...bsa
